The present invention relates generally to motor control systems. More particularly, the invention relates to a motor control system that reduces acoustic noise in brushless DC motors by forcing subsequent phases of the motor to share a fixed current at commutation.
The development of technology in the office automation industry has often been driven by the need for quieter equipment. Quieter equipment typically requires quieter components and underlying systems. For example, the standard office photocopier will have thousands of components, and a large majority of these components will be candidates for reducing acoustic noise emanating from the copier. Another driving force behind the advancement of technology in the office automation industry is cost. It is easy to understand that an ongoing demand for less expensive equipment operating at the same quality level determines the extent to which noise can be reduced.
Motors are used in all types of office equipment for a seemingly endless number of purposes. While AC motors generate relatively small amounts of acoustic noise and are sometimes used in office equipment, a number of factors limit their attractiveness. For example, the relatively high power consumption of AC motors typically requires larger, more expensive power supplies. Furthermore, the office automation industry often requires efficient operation and dynamic control. As will be discussed in greater below, efficient operation and dynamic control are generally more difficult to achieve with AC motors due to synchronization issues. Thus, while AC motors can often be driven at relatively low noise levels, additional requirements such as power consumption and synchronization may dictate the use of different types of motors.
Three phase brushless DC (BLDC) motors can be very quiet when driven with a control capable of supplying sine wave currents to the motor. The reduction in noise is largely due to the fact that as each phase is excited, there is a smooth transition between minimum phase current and maximum phase current (and vice versa for de-excitation). It is in fact the smooth changing of currents and resultant forces that keeps the motor from resonating and thereby generating noise. Simply put, the motor (a resonating mechanical structure) only receives mechanical excitation at the frequency of the sine wave and therefore generates less acoustic noise.
On the other hand, driving BLDC motors with sine wave currents typically results in the synchronization and power consumption problems discussed above for the case of AC motors. This is important because in the office automation industry, synchronization is often paramount. Synchronization essentially involves matching the phase currents (and resulting phase excitations) with the speed of the rotor. Thus, the complicated current transitions associated with sine wave currents can be quite difficult to predict and control when dealing with a high speed motor. To further complicate matters, dynamic speed changes (often occurring between commutation steps) make the task of synchronizing the sine wave currents to the rotor even more difficult.
Driving the BLDC motor with square wave currents, on the other hand, resolves many of the above synchronization issues. Square wave currents have sharp rising and falling edges and are much easier to predict and control. Furthermore, square wave drive systems typically use three hall effect sensors to pick up the signals necessary to commutate the motor. Thus, the hall effect sensors provide a reliable mechanism for detecting speed changes between commutation steps. Since the synchronization benefits associated with square wave control translate into cost and size improvements, conventional motor control systems supply square wave currents to the motor.
In such cases, however, the acoustic benefits provided by BLDC motors are reduced because the rising and falling edges of the square wave currents produce acoustic noise in the motor at the commutation frequency. As already mentioned, acoustic noise is heavily dependent upon the excitation frequencies associated with the drive current. The frequency content of square wave currents is in fact very frequency xe2x80x9crichxe2x80x9d. More importantly, the rising and falling edges of the square wave currents are the primary source of these additional frequencies. It is also important to note that in addition to an increased number of excitation frequencies, the amplitude of these frequencies also increases under traditional square wave control. It is therefore desirable to provide a motor control system that produces the acoustic noise reduction benefits associated with sine wave currents, and the reduced synchronization costs associated with square wave currents.
The above and other objectives are provided by a system and method in accordance with the present invention for controlling a motor. The control system includes a voltage source for providing a DC bus current, and an inverter. The inverter has a switching circuit for regulating the DC bus current to a fixed level. The switching circuit also forces consecutive phases of the motor to share the bus current at commutation. Forcing consecutive phases of the motor to share a desired amount of the DC bus current creates a xe2x80x9croundingxe2x80x9d effect on the phase currents, which results in reduced acoustic noise.
Further in accordance with the present invention, an inverter for a motor control system is provided. The inverter has a plurality of transistors, and a control module. The control module selectively engages the transistors such that each phase of the motor has a phase turn on point that occurs before a phase turn off point of a preceding phase. The control module also pulse width modulates the transistors such that the DC bus current is regulated to the fixed level.
In another aspect of the invention, a method for controlling a motor includes the step of determining a fixed level for a DC bus current. The DC bus current is then regulated to the fixed level, and consecutive phases of the motor are forced to share the bus current at commutation.